Congenital malformations are the leading cause of perinatal mortality and morbidity in the United States and other developed nations. cytogenetic abnormalities, most commonly, the liveborn trisomies 13, 18, and 21, 47, XXY, and 45, X are the underlying basis for many of these malformations and mental retardation. Currently, prenatal genetic diagnosis for these conditions is only routinely offered to pregnant women over the age of 35. Procedures such as amniocentesis and chorionic villus sampling, needed to perform cytogenetics analysis, involve a risk of miscarriage of about 0.5 to 3.0 percent. Clearly, a major need exists to 1) develop a form of prenatal cytogenetic diagnosis without risk to the fetus and 2) develop a test that could be offered to younger pregnant women, who give birth to the majority of infants affected with aneuploidy. We have developed methods of identifying and flow sorting nucleated erythrocytes (NRBC) of fetal origin in venous blood samples from pregnant women. In preliminary experiments, we have employed the technique of fluorescence in situ hybridization (FISH) to demonstrate the presence of trisomy 21 in NRBC isolated from the blood of pregnant women whose fetus had Down syndrome. The proposed experiments will determine whether fetal NRBC in the maternal circulation are a clinically useful source of fetal material for the detection an aneuploidies that cause developmental delay or mental retardation. Fetal NRBC will be identified by monoclonal antibodies and sorted by a flow cytometer in specific groups of pregnant women at high risk for an aneuploid fetus. The resources of the Molecular Cytogenetics Core Facility will be utilized to perform FISH on interphase nuclei obtained from isolated fetal cells. Specific probes for chromosomes 13, 18, 21, X, and Y will be used for aneuploidy detection. The results of our noninvasive studies will be compared to other fetal diagnostic or pathologic studies. The results of the proposed experiments will determine whether a completely safe, rapid, and widely applicable prenatal diagnostic test for fetal aneuploidy can be offered to all pregnant women.